Flexographic printing is a kind of letterpress and has an advantage of being applicable to various printing media because a soft material such as rubber or a synthetic resin is used for a printing plate.
With respect to the printing plate used for flexographic printing, there have been a conventional method of making the plate using a negative film and a computer-to-plate technique (hereinafter, CTP technique) in which information processed on a computer is directly drawn on the printing plate to make a relief, and the CTP technique has been becoming the mainstream in recent years.
An original plate of the flexographic printing plate has a different plate configuration between the case where the negative film is used and the case where the CTP technique is used. The case where the negative film is used and the case where the CTP technique is used are common in that a photosensitive layer comprising a photosensitive resin composition is provided on a substrate of a PET resin or the like, but the layers formed on the photosensitive layer are different between the case where the negative film is used and the case where the CTP technique is used. Firstly, when the negative film is used, a transparent image carrier layer for protecting the negative film from the tackiness of the photosensitive layer exists, and the negative film is closely adhered on the transparent image carrier layer before use, the photosensitive layer is irradiated with an ultraviolet ray from above the negative film, and an uncured portion is then removed to form a relief image. On the other hand, when the CTP technique is used, an infrared ablation layer is stacked on the photosensitive layer, an image is directly drawn on the infrared ablation layer with laser or the like, the photosensitive layer is then irradiated with an ultraviolet ray, and an uncured portion is removed to form a relief image.
Recently, the uncured portion may be removed with a developing solution comprising water as the main component (subjected to water development) to form a relief image.
In an aqueous developable photosensitive resin printing original plate for flexographic printing, a photosensitive layer comprising a photosensitive resin composition obtained by mixing a hydrophilic copolymer, a hydrophobic resin such as an elastomer, a polymerizable unsaturated monomer, a photopolymerization initiator, and the like is generally formed on a support for keeping dimensional accuracy.
In the photosensitive resin composition to be used for such a photosensitive layer, it is preferable that each component in the composition be finely and uniformly dispersed in order to obtain a required image faithfully, and it is preferable that the development rate be fast in order to shorten the plate making time. Further, after a plate is made, the plate needs to withstand the use of aqueous inks, the inks adhered to the plate need to be washed off with water, and therefore it is preferable that the water swelling rate after curing be low, and, as a matter of course, the printing durability be high.
As a water developable photosensitive resin composition, for example, a photosensitive resin composition comprising water-dispersed latex, a photopolymerizable monomer, rubber, and a photopolymerization initiator is disclosed in Patent Literature 1.
In addition, the photosensitive resin composition further comprising a sulfonic acid-based surfactant, wherein the rubber forms a dispersed phase having a size of 10 μm or less, is disclosed in Patent Literature 2.
Furthermore, a photosensitive resin composition wherein the water-dispersed latex is synthesized through emulsion polymerization using a reactive emulsifier containing an unsaturated double bond and is internally crosslinked is disclosed in Patent Literature 3.